The present invention relates to separation of one or more selected components from a stream of fluid containing a plurality of components. More particularly the invention relates to apparatus and methods for removal of selected components from a fluid stream by decreasing the temperature of the fluid to below a selected temperature at which one of condensation and solidification of the selected components occurs thereby forming particles of the selected components, and separating the particles from the stream. Such separation apparatus and methods have application in various processes, for example in drying and removal of nitrogen from natural gas, removal of noxious components from flue gas, in air-conditioning (water removal), and in concentrations or enriching vapors in front of condensors.
Numerous methods and apparatus exist for separating components from a fluid flow containing gases, liquids and/or solids. Conventional separation apparatus include distillation columns, filters and membranes, settling tanks, centrifuges, electrostatic precipitators, dryers, chillers, cyclones, vortex tube separators, and adsorbers. There are disadvantages associated with each of these conventional apparatus which make them undesirable for certain applications.
For example, distillation columns, electrostatic precipitators and dryers are generally large in size, have long residence times, and require high energy input. In addition, these devices are relatively ineffective in separating gaseous mixtures.
Filtration and membrane separation of solid particles from a fluid includes the removal of particles from the fluid by use of a filter or membrane specifically tailored to remove particles from the fluid while allowing the fluid to pass through the filter or membrane. Thus, filters and membrane separation requires that the membrane, filter cake or other similar filtration aid be regenerated or discarded after separation adding increased costs to the process. Additionally, filters and membranes have a long residence time. Settling tanks also have a long residence time and often require additional treatment, such as filtration or centrifugation.
Centrifuges and cyclones both use centrifugal force to achieve separation. Centrifugal separators can achieve separation of immiscible or insoluble components from a fluid medium; however, centrifugal separators require mechanical acceleration of up to 20,000 G. The mechanical parts and energy needed to achieve these velocities make centrifugal separators costly to operate to effectively remove components from a fluid. Cyclones are used to separate gaseous components from gas-liquid fluid flows by way of turbulent vortex flow. Vortices are created in a fluid flow so that heavier particles and/or liquid droplets move radially outward in the vortex, thus becoming separated from gaseous components. Considerable external energy must be added to cyclones to achieve effective separation.
Apparatus and processes exist for creating droplets from a fluid, which are then separated from the fluid. Examples of such apparatus include chillers, throttling valves, turboexpanders and vortex tube separators. Chillers create droplets and may also create hydrates which can clog downstream flow systems.
A turboexpander is an apparatus which reduces the pressure of a feed gas stream. In so doing, useful work may be extracted during the pressure reduction. Furthermore, an effluent stream may also be produced from the turboexpander. This effluent may be passed through a separator or distillation column to separate the effluent into a heavy liquid stream. Turboexpanders utilize rotating equipment, which is relatively expensive. Such equipment requires a high degree of maintenance and, because of the moving parts, has a higher incidence of mechanical breakdown.
In addition, turboexpanders are poorly suited for certain applications, such as for feed gas streams with entrained water.
Vortex tube separators are devices for chilling gas by expansion. A gas is introduced into the vortex tube separator through a header across tangential inlet nozzles. The gas may reach near sonic velocity as it passes into the vortex tube. Condensation occurs during the near adiabatic expansion of the gas. The condensate is forced toward the outer wall of the vortex tube. Simultaneously, gas moves from the wall to the center of the tube. By removing the liquid phase from the tube wall it is possible to separate the gas and liquid phases. Vortex tube separators are not particularly efficient and the fluid flow is limited to subsonic velocities.
Japanese Patent Application No. 63-165849 refers to the separation of a gaseous mixture through the use of supersonic flow. The device includes a subsonic swirler positioned upstream of a supersonic nozzle. The swirling fluid stream passes through an axially symmetric expansion nozzle to reach supersonic velocity and form fine particles. In order to separate a component from the gas flow, a large upstream swirl must be initially provided by the swirler and a significant amount of energy therefore must be input to the system. The system undergoes a large pressure drop and an oblique shock wave occurs downstream after the separation.
U.S. Pat. No. 3,559,373 (Garrett) refers to a supersonic flow separator including a high pressure gas inlet, a rectangularly-shaped throat, and a U-shaped rectangular-cross sectional channel. The channel includes an outer curved permeable wall. A gas stream is provided to the gas inlet. The gas converges through the throat and expands into the channel, increasing the velocity to supersonic. The expansion of the flow in the supersonic region results in droplets which pass through the outer permeable wall and are collected in a chamber. The force available to separate out the droplets is dependent on the radius of the curvature of the channel. The radius of the curvature of the channel, however, must be limited to prevent undesirable shock waves. Therefore, the U-shaped configuration limits the force available for separating out liquid droplets from the flow stream. Further, liquid droplets are collected across only a limited area of the channel.
European Patent Publication No. 496,128 refers to a method and device for separating a gas from a gas mixture. The device includes a cylinder which converges to a nozzle. Gas enters an inlet port of the cylinder at subsonic speeds, flows through a converging section of the nozzle and then out of a diverging section at supersonic velocity. A pair of delta-shape plates arranged in the gas flow generate a vortex. The combination of the supersonic velocities and the vortex allow for condensation and centrifugal force to move a condensed component to an edge zone of the cylinder. An outlet pipe is positioned centrally within the cylinder to allow discharge of the gaseous components of the flow stream at supersonic velocity. The condensed component continues on through a second diverging section, which drops the velocity to subsonic, and through a fan, ultimately exiting the cylinder through a second outlet. The device includes some inherent flaws which inhibit its ability to effectively separate components. Specifically, the change in temperature experienced by the flow stream in the supersonic region over time is too great to grow large particles and therefore the gaseous component of the flow stream still contains substantial amounts of fine liquid particulates. Further, discharge of the gaseous components occurs at supersonic velocities, and thus no final controlled shock wave is utilized to assist in separation.
What is needed is a separation apparatus and method that provides high separation efficiency while avoiding or minimizing pressure drop, maintenance costs, and the need to supply external energy.
The present invention provides an apparatus and method to separate one or more selected components from a compressible fluid containing a plurality of components. The term xe2x80x9ccompressible fluidxe2x80x9d herein shall be understood to mean any gases, gas-liquid mixtures, liquids near their bubble point or dew point, emulsions, and any combination of any of the foregoing, so long as the fluid is sufficiently compressible such that it can be propelled to supersonic velocity via expansion. Preferred compressible fluids include, for example, natural gases, flue gases, and air-water mixtures. The expanding fluid reaches extremely low temperatures, which results in a condensation product. This condensation product can include water and heavier hydrocarbons which must be removed from natural gas if contractual sales specifications are to be met.
The present invention provides a supersonic separator apparatus and method having an improved supersonic nozzle for forming particles of a separable size. The apparatus further includes an intermediate portion through which a compressible fluid stream flows at supersonic velocity and, preferably, a structure for imparting a tangential component to the fluid stream, as well as a collection mechanism to extract one or more components from the fluid stream.
In a preferred embodiment, the mixture of gas and liquid or other droplets or particles enters a swirl imparting section at supersonic velocity. The swirl imparting or intermediate section of the conduit may include a wing. On the leading edge of a preferred wing profile, a strong vortex is generated, forcing the condensed particles toward the wall of the conduit. A liquid film is built up against the inner wall moving helically due to shear forces acting between the swirling gas and the liquid film. The liquid film is transported outside the main conduit via a liquid drainage system, which can be, for example, a co-axial tube or perforations or slits in the conduit.
Although this invention is generally described in terms of the separation of liquid particles (droplets), the invention is not so limited. It is to be understood that the invention may be utilized to separate solid particles from a fluid stream. U.S patent application Ser. No. [09/223,886], filed on even date herewith now U.S. Pat. No. 6,280,502 and of which the entire contents are incorporated herein by reference, discusses the separation of solid particles from compressible fluid streams.
Applications for the present invention include the oil and gas industry, including LPG, LNG, sour gas treatment, downhole and subsea applications, and also, for example, air-conditioning, (convection) drying processes (for e.g., paper, textile, and food processing industries), dust removal, heat pumps (energy savings) and the removal of CO2, N2, NOx, H2S and other materials from flue gases. U.S. patent applications Ser. Nos. [09/223,885 and 09/223,887], both now abandoned both filed on even date herewith and of which the entire contents of both are incorporated herein by reference, discuss, respectively, application of the present invention in downhole and subsea wellhead applications.
The present invention also provides an apparatus and method for capturing a shock wave and for enforcing and strengthening a vortex swirl flow. Additionally, the present invention provides arrangements for extracting particles, enhancing drainage, liquid production, and pressure recovery.
According to a particularly preferred embodiment of the present invention, the separation apparatus includes various structures for imparting of tangential component to the axial velocity of the fluid stream. As an input fluid stream enters a conduit according to several embodiments of the invention, its velocity is substantially in an axial direction. As the fluid stream increases to supersonic velocity in a supersonic passageway of the conduit, it comes in contact with and flows over a wing or other swirl imparting structure. The swirl imparting structure causes the stream to alter its direction tangentially and begin to swirl through the remainder of the supersonic passageway. The velocity of the swirl flow in the supersonic passageway is still supersonic, and there remains a supersonic axial velocity.
Another preferred embodiment of the invention is the creation of a final shock wave in a deceleration zone of the conduit. When fluid flow passes from supersonic to subsonic velocity, a shock wave is created. The supersonic velocity can be reduced to subsonic by any suitable method or structure for causing a shock wave. Preferably, the shock wave is a controlled, final shock wave. A controlled shock wave is one which occurs as a matter of design intent and based upon the geometry of the conduit. A final shock wave is one in which the velocity downstream of the wave is subsonic. Preferably the shock wave is created by inducing the stream of fluid to flow through a deceleration zone. A suitable deceleration zone is a diffuser, and suitable diffusers include subsonic and supersonic diffusers.
As the swirl flow enters a deceleration zone, and consequently decreases its axial velocity to the subsonic, the axial velocity component of the swirl flow diminishes. Thus, the axial velocity of the fluid stream decreases and the tangential velocity remains the same (or decreases to a smaller degree), so the swirl ratio, defined as the Vtan/Vaxial increases.
The present invention achieves enhanced separation of particles or droplets drifting in a vortex flow by, among other things, increasing the swirl ratio. In order to increase the swirl ratio within the supersonic vortex flow, the axial velocity component is decelerated. Just after the shock wave, the swirl ratio reaches its maximum. Separation efficiency is improved if collection of the particles takes place after the shock wave, i.e., in subsonic flow rather than in supersonic flow. The shock wave dissipates a substantial amount of kinetic energy of the fluid stream and thereby reduces the axial component of the fluid velocity while the tangential component either increases or remains substantially unchanged. As a result, the density of particles in the radially outer section of a collection section downstream of the deceleration zone is significantly higher than in the central portion of the conduit. Further, it is now possible for very small particles (less than one micron in size) to be separated. These effects are facilitated by the increased swirl ratio and a reduced tendency of the particles to be entrained by a central xe2x80x9ccorexe2x80x9d of the stream, so that the particles are allowed to agglomerate in the radially outer section of the collection section, from which they can be subsequently extracted.